boost/detail/is_incrementable.hpp

// Copyright David Abrahams 2004. Use, modification and distribution is
// subject to the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
#ifndef IS_INCREMENTABLE_DWA200415_HPP
# define IS_INCREMENTABLE_DWA200415_HPP

# include <boost/type_traits/remove_cv.hpp>
# include <boost/mpl/bool.hpp>
# include <boost/detail/workaround.hpp>

namespace boost { namespace detail { 

// is_incrementable<T> metafunction
//
// Requires: Given x of type T&, if the expression ++x is well-formed
// it must have complete type; otherwise, it must neither be ambiguous
// nor violate access.

// This namespace ensures that ADL doesn't mess things up.
namespace is_incrementable_
{
  // a type returned from operator++ when no increment is found in the
  // type's own namespace
  struct tag {};
  
  // any soaks up implicit conversions and makes the following
  // operator++ less-preferred than any other such operator that
  // might be found via ADL.
  struct any { template <class T> any(T const&); };

  // This is a last-resort operator++ for when none other is found
  tag operator++(any const&);
  tag operator++(any const&,int);

# if BOOST_WORKAROUND(__MWERKS__, BOOST_TESTED_AT(0x3202)) \
    || BOOST_WORKAROUND(BOOST_MSVC, <= 1300)
#  define BOOST_comma(a,b) (a)
# else 
  // In case an operator++ is found that returns void, we'll use ++x,0
  tag operator,(tag,int);  
#  define BOOST_comma(a,b) (a,b)
# endif 
  
  // two check overloads help us identify which operator++ was picked
  char (& check(tag) )[2];
  
  template <class T>
  char check(T const&);
  

  template <class T>
  struct impl
  {
      static typename remove_cv<T>::type& x;

      BOOST_STATIC_CONSTANT(
          bool
        , value = sizeof(is_incrementable_::check(BOOST_comma(++x,0))) == 1
      );
  };

  template <class T>
  struct postfix_impl
  {
      static typename remove_cv<T>::type& x;

      BOOST_STATIC_CONSTANT(
          bool
        , value = sizeof(is_incrementable_::check(BOOST_comma(x++,0))) == 1
      );
  };
}

# undef BOOST_comma

template <class T>
struct is_incrementable
  : mpl::bool_< ::boost::detail::is_incrementable_::impl<T>::value>
{
};

template <class T>
struct is_postfix_incrementable
  : mpl::bool_< ::boost::detail::is_incrementable_::postfix_impl<T>::value>
{
};

}} // namespace boost::detail

#endif // IS_INCREMENTABLE_DWA200415_HPP

Boost C++ Libraries

boost/detail/is_incrementable.hpp